The Trillionth Tonne

Communicating climate risk to non-specialists is not easy. Nonetheless, I think it is possible. In my own personal journey to understanding the risks my family and myself face, I have found that getting to grips with the idea of a carbon budget has been vital. So I have a great deal of gratitude to those scientists who have thought long and hard about how to highlight the link between carbon and temperature change.

The carbon budget concept first found a wider audience in the journal Nature with the publication of two papers led by Myles Allen et al here, and Meinshausen et al here. A less technical commentary piece entitled “The Exit Strategy” also accompanied these two papers and is an absolute must-read for any thinking person.

The central tenet behind these papers is that only a limited amount of fossil-fuel carbon can burnt and turn into CO2 before we are committed to warming the earth by 2 degrees Celsius. Given our current state of knowledge, Myles Allen and his colleagues also suggest that our current carbon budget is one trillion tonnes (or rather this is their best estimate of what can be released). The time path over which that trillion tonnes of carbon is emitted has almost no bearing on the level of actual warming due to the lags of temperature change to CO2 and the fact that CO2 resides in the atmosphere for so long (click for larger image).

Note that they tackle the question of climate sensitivity to CO2 somewhat differently from the approach taken by the Intergovernmental Panel on Climate Change (IPCC) . In short, the IPCC defines climate sensitivity as the rise in global mean temperature based on a doubling of atmospheric CO2 from pre-industrial levels. The preferred metric of Allen and his colleagues is how much global mean temperature rises per one trillion tonnes of carbon.

Helpfully, Oxford University hosts a web site based on this methodology telling us how far we are along the way to burning that trillionth tonne. The answer is here:

This figure is updated in real time here. You can also adjust how much warming you are prepared to accept and how sensitive you believe the climate is to the burning of each trillion tonnes of carbon:

Based on 2 degrees Celsius of warming and a 2 degree sensitivity to one trillion tonnes of carbon, the site estimates that the trillionth tonne of carbon will be emitted in August 2041. Further, to prevent an overshoot of the trillion tonne target requires a 2.4% per annum decline in emissions starting immediately.

Even more alarming, if we are unlucky and climate sensitivity comes in at the higher end (under their methodology that means 3 degrees Celsius of warming for every 1 trillion tonnes), then obviously only 666 billion tonnes can be emitted to stay below 2 degrees. That 666 billion budget will be used by March 2021 on the current emissions path.

This methodology allows a more realistic approach to scenario analysis than that applied up to now by the Intergovernmental Panel on Climate Change. The worst-case scenario in the Fourth Assessment Report (AR4) was called A1FI and envisaged a world of high growth, catch up by the emerging economies and a continued reliance on fossil fuels. The only silver lining in respect of this grim scenario is that it is very unlikely that such a pure exponential curve can be maintained.

A number of feedbacks would likely kick in to break this exponential curve. Stuart Staniford at his Early Warning blog looks at a ‘Panic and Repent’ scenario where the world suddenly wakes up to the existential threat posed by climate change and mounts a WW2 style renewable response.

Alternatively, I could see a scenario whereby the global elites persist in their folly but, nevertheless, the damage from climate change starts to beat down GDP growth and so snaps the A1FI exponential curve; in short, growth is bludgeoned over the head by the baseball bat of anthropogenic global warming (AGW) until it submits. Or, perhaps, the curve dies a natural death in the face of Robert Gordon’s ‘end of growth’ scenario. Whatever happens, emissions don’t blast off to ‘infinity and beyond’ Toy Story style as at some stage a negative feedback will kick in.

That is the good news. The bad news is that such fulcrum points may arrive too late. The persistency of CO2 in the atmosphere and its lagged effect mean we have a horrible degree of lock-in. Nonetheless, the carbon budget can tell us what the probabilistic consequences are of shooting past a trillion tonnes. I will take a further look at the ‘panic and repent’, ‘bludgeoned by the AGW baseball bat’ or ‘end of growth’ scenarios in a future post.